1. Field of the Invention
This invention related to a flame spray tape to mask articles during a plasma spray process.
2. Description of the Prior Art
Plasma or flame spraying of parts is a known technique for applying a protective metal or ceramic coating to the part. Such process provides a thermal spray coating over the part by bringing the metal or ceramic to the melting point and spraying on a surface to produce a thin coating. Plasma spray coating typically is achieved using a plasma gun or similar device.
In the plasma spray process, it is important to mask certain areas of the parts in order to prevent application of the coating. Reasons for masking parts include preventing the coating from entering apertures in the part, maintaining dimensions within a critical range, weight savings and the like. To achieve such masking, a masking tape is applied over the areas in which the coating is not desired.
The masking tape must exhibit excellent thermal and abrasion-resistance, both in protecting adjacent surfaces from the grit blasting that is typically used as a surface preparation and the actual plasma spray coating. Such tape must not lift off or fray during this demanding process and are designed to quickly and easily release from the part surface without leaving an adhesive residue.
Conventional plasma spray tapes typically include a glass fabric which is impregnated with a silicone adhesive and which is coated with a high temperature silicone pressure sensitive adhesive. A release liner is usually employed for convenient handling. Other types of masking tapes include a thin aluminum foil laminated to a fiber glass cloth.
Although such masking tapes are effective with the typical plasma spray process, they are not effective with a recently introduced, more demanding process known as a high velocity oxy-fuel (HVOF) process. This process is a continuous combustion process in which the spray gun is essentially a rocket in which the powder is injected into the exhaust stream. The exhaust stream is exiting at hypersonic speed (several thousand feet per second).
The above-referenced patent application describes a masking tape for the HVOF process which includes an inner fabric layer formed of aramid fibers, a silicone rubber impregnating the fabric, a first silicone rubber layer coated onto one face of the impregnated fabric, a second silicone rubber layer coated onto the other face of the fabric, and a silicone pressure-sensitive adhesive coated onto the second silicone rubber layer. It has been discovered that the peel strength of such a tape to a supporting structure (e.g., the part to be coated) is not maintained over time. As a result, in some situations such tape may not be capable of adhering to the part to be protected and withstanding many passes of the HVOF gun.
It would be desirable to provide an improved plasma spray masking tape which retains its peel strength and will withstand the extremely demanding HVOF process.